1. Field of the Invention
The present invention relates to an image formation apparatus.
2. Description of the Related Art
With image formation apparatuses (image recording apparatuses) employing image formation processing such as electrophotography, electrostatic recording, or the like, an electrostatic latent image is formed on a photosensitive drum or the like serving as an image carrying member, the electrostatic latent image is developed with a developing agent so as to visualize the image as a toner image, this toner image is transferred onto a recording medium such as a sheet, following which the recording medium on which the toner image is transferred is passed through a nip portion made up of a fixing roller and a pressure roller provided on a fixing device, thereby thermally fixing the toner image on the recording medium as a permanent image.
FIG. 13 is a schematic side view of primary parts illustrating an example of such a typical image formation apparatus. A photosensitive member 1 (hereafter, referred to as a “photosensitive drum”) is disposed above the face of a sheet and rotates in an X direction shown with an arrow, and the surface thereof is uniformly charged by a charging roller 2 connected to a high-voltage power source 7, and a laser beam L modulated by image signals from a laser scanner 3 is cast onto the charged face thereof so as to form an electrostatic latent image. Toner 5 is supplied onto the latent image from a developing device 4 so as to form a toner image. The toner image reaches a transfer nip portion T.
The transfer portion T comprises a nip portion between the photosensitive drum 1 and an electroconductive transfer roller 6 in contact with the photosensitive drum 1. Synchronously with the timing of a toner image portion on the photosensitive drum 1 reaching the transfer portion T, a recording medium P is supplied and passed through the nip portion. At this time, a transfer bias is applied onto the transfer roller 6 by a power source 8, and the toner image on the photosensitive drum 1 side is transferred onto the recording medium P, following which the recording medium P holding a toner image leaves the transfer portion T and is transported to an unshown fixing device.
Recently, film-heating fixing devices have been proposed which employ a fixing method wherein power is not supplied to the fixing device in particular during a standby period, so as to suppress power consumption as much as possible (e.g., Japanese Patent Laid-Open Nos. 63-313182, 2-157878, 4-44075, and 4-204980).
The fixing device comprises a heater, a slidable thermostable film (fixing film), and a pressure member for forming a fixing nip portion by coming into contact with the heater across the film, and is a device for nipping a recording medium on which an unfixed image is formed between the film of the fixing nip portion and the pressure member so as to transport the recording member, and for fixing the unfixed image onto the recording medium as a permanent image by heat provided from the heater through the film and pressure force of the fixing nip portion.
With such a film-heating fixing device, a low-thermal-capacity linear heating member serving as a heater and a thin-low-capacity member serving as a film can be employed, thereby enabling power to be conserved and wait periods to be reduced (meaning quick starts).
With this type of film heating fixing device, there are two driving methods for the fixing film. One is a tension providing method wherein, while providing tension on the fixing film with a dedicated transporting roller and a dedicated driven roller, the fixing film is transported between the driven roller and a pressure roller serving as a pressure member. The other is a tensionless method wherein a cylindrical fixing film is driven with the transporting force of the pressure roller by rotating and driving the pressure roller serving as a pressure member. The former has an advantage of improving transportability of the fixing film, and the later has an advantage of realizing a low-cost device owing to simplification of the device configuration.
In recent years, demand for printers have increased with the development of the computer industry, so printers have come to be widely used worldwide. Thus, in accordance with sheets having various kinds of thickness and various surface properties, and speeding up of image formation apparatuses, satisfied fixability has been obtained by gradually increasing the momentary amount of heat which is provided to a recording medium from the heater so that the period required for printing the first sheet can be reduced and printing fixability of the first sheet can be ensured. Further, in recent years, in response to increased user requests for high quality images, printers with excellent performance in dot reproduction and gradients have been released, which enable high quality images to be printed by further reducing the grain diameter of toner serving as an image manifesting agent (a developing agent).
Moreover, with regard to transfer bias control for these types of image formation apparatuses, the ATVC method (Active Transfer Voltage Control) has already been proposed (for example, Japanese Patent Laid-Open No. 2-264278).
This method is a way to optimize a transfer bias applied onto the transfer roller 6 at a transfer period, and optimizes the transfer bias with an arrangement wherein a desired constant current bias is applied onto the photosensitive drum 1 from the transfer roller 6 during the initial rotation of the image formation apparatus, the resistance of the transfer roller is detected from a detected voltage Vo at that time, and the constant voltage bias corresponding to the resistance is selected in a transfer period. In this case, the transfer voltage Vt is represented by the following expression (ATVC expression).Vt=AVo+B (wherein A and B are constants)
With a contact transfer method, the optimal voltage to be applied at the leading edge of a recording medium changes depending on the resistance of the transfer roller. Moreover, the transfer roller has irregularities in resistance over a wide range, and the properties of the recording medium drastically change from high-temperature and high-humidity (H/H) environments to low-temperature and low-humidity (L/L) environments. Taking advantage of these properties allows the apparatus to distinguish H/H environments from L/L environments based on the resistance of the transfer roller, and also enables the same transfer performance to be maintained by the transfer roller even if environmental variations occur.
The Vt is calculated from Vo obtained at the initial rotation in order to prevent developing on the leading edge of a recording medium from a substandard transfer current such as an explosive image.